Tissue is a assemblage of cells and cell products that have a common function.
During embryonic development animal cells differentiate and become integrated into four basic tissue types: epithelial, connective, muscular, and nervous tissues, each an assemblage of similarly specialized cells united in performing specific functions.
The basic tissues, each containing extracellular matrix (ECM) as well as cells, associate with one another in the variable proportions and morphologies characteristic of each organ.